1. Field of the Invention
This invention relates generally to the field of echo control devices and more particularly to the field of high speed analog echo control devices such as echo cancellers.
2. Background of the Invention
On the past there have been basically two approaches to providing control over echoes for local telephone loops. The analog techniques generally involved use of various filtering arrangements which essentially provided impedance matching to the telephone line in order to reduce echoes. Such filtering schemes however are somewhat temperamental and are not readily adaptive to the frequent changes to the impedance characteristics of the transmission line. For example, temperature and humidity as well as wind, rain, snow and changing line terminations can all affect the impedance characteristics which are being corrected by a typical analog echo canceller. It is, therefore, very difficult to accommodate the very wide range of impedances which may be seen by such an echo suppressor, since a wide variety of impedances may be present on a transmission line. Since it is desirable to provide echo suppression in the order of 50 to 60 DB, analog echo suppression devices have been largely abandoned in favor of more effective digital techniques.
In a digital echo canceller, a representation of an echo signal is normally stored in memory and appropriately deleted mathematically from incoming data. Digital echo cancellers are able to effectively provide the required attenuation of echo signals but have drawbacks of their own. For example, in order to achieve a very high degree of echo cancellation, such digital echo cancellers require a highly accurate analog to digital converter and digital to analog converter. Such echo cancellers also require use of a reasonably powerful processor to manipulate the echo signal and data signal appropriately to achieve echo cancellation. While in general this is not a disadvantage for typical low speed data lines, owing to the recent advances in these technologies which allow for reasonably inexpensive computer elements, such items represent a major cost in higher speed environments.
In order to achieve approximately 50 to 60 DB of echo cancellation an analog-to-digital converter and digital to analog converter must have at least 12 bits of accuracy. For a digital echo control device to operate above approximately 50 to 100 KBPS, the cost of such high accuracy digital to analog converters which wil function at this data rate becomes prohibitive. Moreover, very high speed processors are also required at such speeds to provide digital echo cancellation. The present invention overcomes these drawbacks in an echo control device whichis capable of operation at extremely high speeds using analog technology. No digital-to-analog or analog-to-digital conversion is required. Microprocessors or microcomputers may also not be required (but may be desirable for various control functions) to provide a high degree of echo cancellation which can readily adapt to changing transmission line conditions.